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The evidence for the existence of electrogenic pumps in higher plant cells is reviewed. A new theory of electrogenic transport is given. Which ion is pumped and the energy source for electrogenic transport are discussed.

Longitudinal electrical resistances have been measured on 2-centimeter segments of corn (Zea mays L.) roots, cut at varying distances from the root apex. The segment resistances vary from 400 to 100 kilohms per centimeter along the root length (apex to 18 cm), with the maximum occurring in the 2-to 4-centimeter segment, and decreasing thereafter toward the root base. Measurements of isolated root cortical sleeves and steles show that the pathway of least resistance is in the cortex, which has a greater cross-sectional area; the specific resistance of the older stele is less than that of the cortex. The anatomical state of the xylem cannot be inferred from electrical resistance determinations.There have been several recent reports of resistance measurements in plant root segments (3, 4), perhaps triggered by the report of Davis (1) that an electric potential difference was maintained between the xylem exudate fluid of an excised corn root and the bathing solution, even when successive segments were removed from the root tip. The resistance measurements were undertaken to try to understand this effect, since the immediate reaction might be that the observed potential difference in the Davis experiment (1) would be short-circuited through the xylem vessels of a root segment open at each end to ionic solution.In fact, the electrical situation is more complex than this, and was analyzed by Ginsburg (2) who realized that a root segment behaves electrically as an extremely leaky cable. The conclusion reached by him was that the longitudinal resistance of a root segment need not be very large, provided that it is appreciably greater than the radial resistance of the segment, for a stable potential difference to be maintained between the ends of the segment.There is a more general physiological point to these considerations. It rests on the question of whether root pressure exudation from the young, absorbing regions of roots takes place through fully mature metaxylem vessels with perforated cross walls, or whether it is generated in the xylem vessels much as turgor is in other cells. This process requires the presence of membrane-bound cytoplasm in the vessels (e.g. ref. 4). This hypothesis is that in the zone of perforation plate formation, the functioning vessels, i.e., the early maturing metaxylem vessels (5-10 cm from the apex in maize roots), retain intact protoplasm with plasmalemma and tonoplast. into the nonliving xylem. In part, resistance measurements have been designed to give evidence on the state of the xylem vessels, but as shown below, it is not possible to infer the anatomical condition of the vessels from resistance measurements. MATERIALS AND METHODSZea mays (cv. Golden Bantam) seeds were surface-sterilized in 1% Clorox solution, thoroughly rinsed in running water, and set to germinate on moist filter paper in the dark at 20 C. After 2 to 3 days the young seedlings were transferred to a hydroponic culture chamber, and continued to grow in aerated lX solution (4) in the-light at 20 C for a furth...

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The rapid reduction in cell electropotentials induced by metabolic inhibitors is strong evidence for an electrogenic ion pump. According to Ohm's law, such a depolarization might be explained by a reduction in electric current, I, with unidirectional transport of a given ion, or an increase in permeability (decrease in resistance). With cells of etiolated seedlings of Pisuml sativum L cv. Alaska and Zea mnass cv. Golden Bantam, carbon monoxide inhibition, which occurs only in the dark and is readily reversed by light, allows repeated cycling of depolarization and repolarization; there is no effect on cell membrane resistance. In contrast, cyanide inhibition results in a marked increase in membrane electrical resistance; with cyanide following repeated pulses of current used in measuring cell membrane resistance, the resistance eventually (about 10 minutes) shows an abrupt drop as in the "punch-through" effect reported by H. G. L. Coster (1965. Biophys. J. 5: 669-686 component of the observed electropotential which is produced by the separation of charge at the pump will immediately become zero, and depolarization of the membrane by that amount will be observed. Although the cyanide effect in higher plants is relatively rapid, it is very much longer than the ATP turnover time in the cell, which is less than 20 sec (2). Slayman et al. (21) have shown spectrophotometrically that after poisoning, ATP concentration in Neurospora crassa decays with a time constant of 5.4 sec.In an attempt, therefore, to find an inhibitor with a poisoning effect which is not rate controlled by diffusion into the tissue, as CN-almost certainly is, we have used solutions saturated with CO to bathe the tissue and have induced inhibition by switching off the light. Since the tissue can be allowed to saturate with CO in the light, there is no diffusional lag at the onset of inhibition which is, therefore, essentially instantaneous. Hopefully, resolution times of the order of milliseconds can be achieved and the effects occurring within the ATP turnover time can be monitored.Carbon monoxide reacts with both Fe and Cu to form carbonyl compounds (11,24,25) but only the iron carbonyl is light-reversible; light at 430 nm appears to be the most active in causing the disassociation of the iron carbonyl (19). Early work showed that CO inhibits respiration by competing with OL for Cyt oxidase, the degree of inhibition being dependent upon the ratio of CO to 02 (14). Weeks and Robertson (26) demonstrated that CO inhibition of respiration in carrot tissue is abolished in the light. The respiration decrease caused by 1 mm CN-(another inhibitor of Cyt oxidase) was greater than that caused by saturating the incubation medium with 95% CO; this difference was attributed to the noncompetitive inhibition exhibited by CN-versls the competitive type assumed for CO (11. 14). Robertson et al. (18) found that CO in the dark strongly inhibited the enhanced respiration induced in carrot discs by 2, 4-dinitrophenol.Carbon monoxide has also been shown to inhi...